1. Field of the Invention
The present invention relates to a polishing machine having a taut microabrasive strip and to an improved wafer support head.
The invention more particularly applies to the polishing of integrated microelectronic components in semiconductor wafers (e.g. of silicon). It can in particular relate to magnetic read-write heads.
2. Discussion of the Related Art
Processes for producing such heads are described in numerous documents and in particular in U.S. application Ser. No. 4,837,924 and U.S. application Ser. No. 4,333,229. The first document relates to so-called horizontal structure heads formed from a stack of layers deposited on the upper face of a semiconductor wafer, while the second document relates to so-called vertical structure heads formed from layers deposited on the edge of such a wafer.
The micromachining operations performed on such wafers consist in the first case of levelling or planarizing and polishing various intermediate subassemblies obtained during the production process, defining a head gap and bringing the complete head into the general plane of the substrate, also known as the movement plane. In the second case, the micromachining operations aim at defining a head gap and at adjusting the shape of the movement shoes.
Although it could possibly apply to the construction of heads of the second category (vertical heads), the machine according to the invention is more particularly intended for the polishing of assemblies or subassemblies corresponding to the first category (horizontal heads), because it is in this case that the technical problems are the most difficult.
FIG. 1 shows as an example of a part to be polished a horizontal structure magnetic read-write head. The assembly shown corresponds to the final stage of production prior to the final polishing. The assembly comprises a silicon substrate 10 in which has been etched a recess, an iron-nickel alloy magnetic circuit 12, a double copper coil 14, a 3 to 6 .mu.m thick silica layer 16, an approximately 1 .mu.m thick amagnetic silica spacer 18 and two iron-nickel upper pole pieces. The plane of the final polishing is indicated in dotted line form and is designated 22.
The removal of the material relates to the pole pieces 20 and extensions 23 made from silica. In order not to bring about a deterioration of the magnetic circuit, the removal must not reduce the thickness of the uniform silica layer by more than 0.3 .mu.m. The final polishing plane defines the movement plane of the head.
Two such heads are generally juxtaposed on parallel strips known as skis defining two movement planes in a generally catamaran-like structure.
The polishing operation, which consists of the removal of a very small material quantity is well known and is encountered in metallography, optics and microelectronics.
It is possible to use one or other of the two following procedures. The first consists of grinding with a diamond tool, where machining leads to a continuous or semi-continuous "shaving" by two relative combined movements between the tool and the part to be machined (an advance movement and a cutting movement). The second consists of grinding and polishing constituting a varyingly fine abrasion or cold-working of a controlled nature of the surface by rubbing on very varied disks, which are not abrasive by nature and to which an abrasive in paste or aqueous solution form has been applied. A variant consists of placing on a rotary polishing disk an abrasive film disk and spraying the latter during polishing with a liquid in order to cool the part and prevent dirtying.
The polishing of semiconductor wafers having a very large number of integrated microcomponents causes various problems. Firstly the wafer is deformed and deformable. The grinding operation must also affect simultaneously several materials of very different hardnesses such as silica, alumina, alumina/titanium carbide alloy and iron/nickel alloy. The parts to be ground have very small surfaces compared with the silicon wafer. Finally, it is a question of machining in their thickness layers deposited on a wafer and it is generally necessary to simultaneously polish 600 excrescences corresponding to 600 magnetic heads, which project by a few microns and this is necessary with an accuracy of approximately 1 nanometer, without reducing the thickness of the film covering the wafer by more than 200 to 300 nm.
The known polishing machines do not make it possible to satisfy all these requirements. In particular, the use of a liquid with abrasive grains or an abrasive film bonded to a support is not suitable, as is apparent in conjunction with FIGS. 2 to 6.
FIG. 2 illustrates the known principle of grinding with a liquid containing abrasive grains. The wafer 10 and its excrescences 25 are positioned facing a polishing disk 23 and the abrasive grain-containing liquid 24 forms a film between the wafer and the reference plane. The wafer translation movement leads to the abrasion of the excrescences.
However, complex hydrodynamic phenomena more particularly associated with the formation of whirling or turbulent movements around the excrescences and cavitation phenomena lead to a defective polishing, whose result is illustrated in FIG. 3. In FIG.. 3, part a shows an excrescence 25a before polishing and whose shape very substantially corresponds to that encountered in the case of integrated magnetic heads, as will be shown hereinafter. This profile assumes the shape 25b after the start of polishing (part b) and finally the shape 25c (part c) at the end of polishing. It can be seen that the sought result has not been obtained, because the movement plane has been reached and peaks remain. In particular, soft materials are hollowed out more than hard materials.
Another known method consists of using a microabrasive plastic film bonded to a reference disk. FIG. 4 shows a reference disk 23 to which a microabrasive film 27 has been bonded by adhesive points 28 (aerosols). The adhesive coating has a thickness of approximately 100 .mu.m. The sheet thickness is approximately 50 to 75 .mu.m. Therefore the assembly has a thickness of approximately 150 to 175 .mu.m.
FIG. 5 shows this abrasive means with a wafer 10 and its excrescences 25 to be polished. It is possible to see the presence of tile excrescences and the relatively great thickness of the polishing layer lead to a corregating or creasing of the latter due to local compression of the sheet and crushing of the adhesive points. Here again the final polishing obtained is not satisfactory. FIG. 6 shows in diagrammatic manner the profile of a polished shoe 29, prior to polishing (FIG. 6a) and after polishing (FIG. 6b).
Polishing machines are also known which make use of an abrasive film, which is not bonded to a reference surface, but is instead held taut above a disk. Such machines are described in DE-U-8 717 353 and DE-OS 26 37 343. These machines comprises a delivery roll or reel and a receiving roll or reel between which passes in a stepwise movement the microabrasive strip. The latter passes above a soft material part. The part to be polished, which is in this case a plate base, is held by a head performing a rotary movement. A pneumatic means located in the lower part of the machine makes it possible to engage the microabrasive strip below the plate base, so that the latter deforms the abrasive film and is embedded in the soft part. The tension of the strip is obtained by means of grippers or jaws, which simultaneously make it possible to bring about the stepwise strip advance.
Such a machine is not suitable for polishing semiconductor wafers for a number of reasons. Firstly, the embedding in the soft material is inadmissible for already indicated reasons, namely the reliefs would be rounded and deformed. Thus, it is necessary to work on a perfectly planar reference disk using a very thin microabrasive film in order to obtain the benefits of the flatness of the reference disk.
Moreover, although the polishing of plate bases allows the use of coarse abrasive grains, the polishing of semiconductor wafers requires much finer grains. However, with such grains, there is the phenomenon of the adhesion of the wafer to the abrasive film to the point that the separation of the wafer at the end of polishing requires the formation of an air wedge to permit the separation of the wafer. This phenomenon tends to cause corrugations on the microabrasive strip. To avoid this risk, it is necessary to make the microabrasive strip very taut over its entire length. However, this is not possible with the machine according to DE-U-8 717 353, which only has for this purpose handles (or jaws) gripping the sides of the strip. These means would lead to a certain tension on the edges of the strip, but not in the center and would also lead to risks of the strip tearing.
Moreover, with the prior art machine, it is not possible to continuously move the taut abrasive strip. Thus, the advance of the strip can only take place stepwise, because it is held taut by members which grip it.
The need to use very fine polishing grains and a very taut film, which leads to the adhesion of the wafer, causes other problems not solved by the machine described in DE-U-8 717 353 and DE-OS 26 37 343. Thus, in the case of such machines, the stone to be polished, namely a plate, is simply held in a support by a vacuum produced above the plate. Such a vacuum would have to be very high in order to hold the semiconductor wafer and would lead to the breaking of the latter.
Finally, a head performing a rotary movement like that of the aforementioned documents would not be suitable for the polishing of semiconductor wafers, because then the center of the wafer would not be polished. A simple rotary movement can only be suitable for circular parts, such as the base of plates.